Implication of choosing powers from another class's list

[ShinRyuuBR]

My personal guess is that the Half-Elf racial power allows you to get one power from a different class of the same power source; that'd keep down the Fighter/Magic Missile problem (and this fits our 1 data point).

I agree. This not only is consistent with Tira, but also with the fact that WotC has said that half-elves are at some kind of advantage to multiclass, compared to the other PH races.